You're referring to the Navajo Code Talkers. The marine corps used native Navajo to convey radio messages at the tactical level later in the war. Messages were transmitted in a special code in the Navajo language. For example, "potato" meant "hand grenade". The Japanese tried to break the code with a Navajo soldier captured in the Philippines early in the war. This man understood the language but, not being schooled in the code, he couldn't adequately decipher messages. This is said to be the only "code" in military history that was never broken by the enemy. It was so effective that it was classified for more than 25 years after the war ended, in the possibility it may have been needed again.